RESEARCH ARTICLE

A ROF system employing FBG and low-cost DML to generateOFDM signals

Hoang Viet Nguyen

Received: 26 July 2013 /Accepted: 8 January 2014 /Published online: 25 April 2014# The Optical Society of India 2014

Abstract We have investigated and demonstrated a novelscheme to generate orthogonal frequency divisionmultiplexing (OFDM) signals for Radio Over Fiber (ROF)systems using a direct-modulation laser (DML). We employthe Fiber Bragg Grating (FBG) because the bandwidth of theoptical modulator is largely reduced and the architecture of theROF system is simpler. The 40 GHz optical mm-wave signalwas generated by a DML driven by the 2.5Gb/s electricalOFDM signals carried by only 20 GHz RF signals. This novelscheme employs a low-cost DML to generate mm-wave sig-nal to carry OFDM signals which is a practical scheme to beapplied for the future broadband access networks.

Keywords Fiber bragg grating (FBG) .Orthogonal frequencydivisionmultiplexing (OFDM) . Direct-modulation laser(DML) . Radio over fiber (ROF) . Optical millimeter wave(mm-wave)

Introduction

Recently, the number of wireless network users is going toincrease strongly due to the emerging of modern applicationsand internet service. The bandwidth requirement to deliver high-speed data in the access network will grow to multi-gigabits persecond in the near future. Optical fiber networks are expected tosatisfy the increasing traffic demand by utilizing the high-capacity and low-cost characteristics. The mm-wave techniqueis considered to be a promising one to satisfy the increasing

demand of broadband wireless communication because of itswide bandwidth. ROF techniques become attractive solutions inrealizing future broadband wireless networks because they havehuge bandwidth and they can be used for the distribution ofwireless signals which grow very fast. For reducing the total costand making simpler configuration of the ROF systems, manyschemes of wavelength reuse or centralized lightwaves in thecentral office (CO) have been proposed and experimentallydemonstrated [1–5], [15–18]. Recently, OFDM signals becomea strong candidate for transmission in the next generation long-haul and access networks because it has high spectrum efficien-cy and the resistance to chromatic dispersion and polarizationmode dispersion [6–14]. Therefore, optical OFDM is naturallysuitable for ROF systems to increase the bandwidth and extendthe transmission distance. The combination of OFDM modula-tion format and ROF technique has been studied widely[19–24]. Many approaches for mixing or up conversion of RFsignals have been introduced such as utilizing optical double-side band (DSB), single-side band, and optical carrier suppres-sion modulation. The main advantage of DSB technique issimple, low-cost and needing only a single-arm modulator. Thisnovel scheme introduces a low-cost solution to generate opticalDSB. This method is based on only one low-cost DML. Whencompared with optical external modulator, the DML is cost-effective and has a higher output power. In this paper, we haveinvestigated and demonstrated a ROF system to transmit2.5Gbit/s 16PSK OFDM signals on 40 GHz mm-wave gener-ated by a DML. We employ the FBG because the bandwidth ofthe optical modulator is largely reduced and the architecture ofthe ROF system is simpler.

System architecture

Table 1 shows the OFDM-ROF system parameters. In thissystem, the expensive broadband external modulator is

H. V. Nguyen (*)School of Computer and Communication, Hunan University,Changsha 410082, Chinae-mail: viet02342000@yahoo.com

H. V. NguyenFaculty of Electronics, Industrial University of Ho ChiMinh City, HoChi Minh City, Vietnam

J Opt (April–June 2014) 43(2):159–164DOI 10.1007/s12596-014-0192-y

removed. A RF(f) clock and OFDM analog data are mixed byan electrical mixer. Then the mixed signals are amplifiedwhich are employed to drive a DML to create optical doublesideband (DSB) signals. We use the FBG as the optical filterof the system. An optical circulator (Cir) and a FBG areemployed to suppress the optical carrier from the first-ordersidebands.

The architecture of a ROF network is shown in Fig. 1. Itconsists of central office (CO), base station (BS) and customerunit. In the CO, we need to generate the optical mm-wavesignals. The length of the transmission fiber from the CO tothe BS can be up to 20 km or more. In the BS, we employ ahigh-speed optical detector to realize optical/electrical (O/E)conversion for the optical mm-wave signals before the con-verted electrical RF signals are boosted by using a high-powerRF amplifier. The RF signals can be broadcasted to the Cus-tomer Unit by antenna.

In the real system, there are some higher harmonics be-cause of the nonlinearity of the DML, but they are too small tobe considered. The 2.5Gb/s baseband signal is mixed with a20 GHz sinusoidal wave to realize sub-carrier modulation(SCM) and then the mixed signals are employed to drive theDML to generate the DSB optical signals. The waveformoutput of the DML which is driven by a mixing RF signalf(t)cosωlt can be approximately represented byG(t) which has

one optical carrier and two first-order sidebands asfollows:

G tð Þ≅G0 J1 ξ f tð Þ½ �cos ωc þωlð Þtþ G0 J0 ξ f tð Þ½ �cosωct

þ G0 J1 ξ f tð Þ½ �cos ωc−ωlð Þtð1Þ

Where f(t)[=0, or 1] is the digital downlink data, ωl is theangular frequency of LO signal. G0 and ωc are the electricalamplitude and the angular frequency of the lightwave outputof the DML, respectively. J0(.) and J1(.) are the zero and thefirst-orders of the Bessel function of the first kind, respective-ly, and ξ is the intensity modulation index.

A Cir and a FBG are employed to suppress the opticalcarrier from the first-order sidebands. We assumed to simplifythe analysis that the optical carrier is completely separatedfrom the first-order optical sidebands.

For transmission over the L-length downlink fiber, β(ω)represents the propagation constant of the fiber. τ+ and τ− are,respectively, the delay deviations caused by the fiber chromat-ic dispersion at the optical sidebands (ωc+ωl and ωc−ωl)from the original optical carrier atωc. They can be expressed asτ+=−2πLcD ωl/ωc

2 and τ−=2πLcD ωl/ωc2, where D is the

chromatic dispersion parameter. The generated optical DSBsignal transmitted over the L-length downlink fiber, due to thefiber chromatic dispersion, the optical DSB signal becomes

G tð Þ ¼ G0 J1 ξ f t−τþð Þ½ �cos ωc þωlð Þt−β ωc þωlð ÞL½ �þ G0 J1 ξf t−τ−ð Þ½ �cos ωc−ωlð Þt−β ωc−ωlð ÞL½ �þþG0 J0 ξf tð Þ½ �cos ωct−β ωcð ÞL½ �

Gup ¼ G0 J1 ξf t−τþð Þ½ �cos ωc þωlð Þt−β ωc þωlð ÞL½ �þ G0 J1 ξf t−τ−ð Þ½ �cos ωc−ωlð Þt−β ωc−ωlð ÞL½ �

ð2Þ

Table 1 OFDM-ROFsystem parameters Parameters Requirement

Carrying signal OFDM signal

mm-wave generation DSB modulation

Optical filter FBG

External modulator Don’t need

Transmission length 40 km

Fig. 1 Configuration ofmm-wave OFDM-ROFsystem

160 J Opt (April–June 2014) 43(2):159–164

Applying this upper branch optical signal to a broadbandphoto-detector (PD), due to square-law detection by the PD, thetwo terms in Eq. (2) beat with each other and generate electricalsignal as follow:

iout ¼ 1

2G2

0 J21 ξð Þ f t−τþð Þ þ f t−τ−ð Þ½ � þ G2

0 J21 ξð Þ f t−τþð Þ

f t−τ−ð Þcos 2 ωlt−β ωc þωlð ÞLþ β ωc þωlð ÞL½ �

We have: β ωc �ω1ð Þ ¼ β ωcð Þ �ω1β0 ωcð Þ þ 1

2ω21β

00 ωcð Þ þ…

The property of J1(ξχ)= χJ1(ξ) (for χ =0, or 1) is used.By expanding the propagation constant of the fiber for each

optical sideband to a Taylor series around the angularfrequency of the optical carrier, the generated electricalsignal becomes

iout ¼ 1

2G2

0 J21 ξð Þ f t−τþð Þ þ f t−τ−ð Þ½ �

þ G20 J

21 ξð Þ f t−τþð Þ f t−τ−ð Þcos2ωl t−β’ ωcð ÞL½ �

ð3Þ

It can be seen from Eq. (3) that the generated electrical

signal mainly consists of the DC component 12G20 J

21 (ξ)[f(t- τ+)+

f(t-τ−)] and the harmonic component at 2ωl. The duty cycle ofthe data code after down-converted from the mm-wave will

Fig. 2 The simulation for theOFDM-ROF system

Fig. 3 Optical spectra at differentlocations labelled in Fig. 2

Fig. 4 Constellation diagrams of the demodulated OFDM signal for different transmission distance

J Opt (April–June 2014) 43(2):159–164 161

reduce. When τ = τ−−τ+ is equal to the duty cycle of thebaseband data, there will be no mm-wave signal at 2ωl, whereτ is the delay time difference between the first-order upper andlower optical sidebands caused by the chromatic dispersion. Thetransmission distance of the optical mm-wave generated by thisscheme is also limited because of the interference to the basebanddata.

The configuration of mm-wave OFDM-ROF system isshown in Fig. 1. At the CO, a low-cost DML is used to generateoptical mm-wave signal. We employ the DML because thebandwidth of it can go up to 30 GHz. We can use an electricalmixer to up-convert the baseband signal. Then, the mixedelectrical signals can be employed to directly drive the DMLto generate optical mm-wave signals. In this way, double-sideband (DSB) modulation signals can be generated. AnRF(f) clock and OFDM analog data are mixed by an electricalmixer. Then the mixed signals are amplified which areemployed to drive a DML to create double sideband (DSB)optical signals. ACir and a FBG are used to suppress the opticalcarrier from the first-order sidebands. Then the first-order

sidebands will be transmitted over SSMF to the base station(BS) after boosted the power by the EDFA. At the BS, theoptical signal after O/E conversion is broadcasted by an antennato the customer unit. TheOFDMbaseband signal is obtained bydown-converting the received signal from the antenna.

Simulation and results

The simulation for this ROF system is shown in Fig. 2. At theCO, we need to obtain 2.5Gb/s electrical OFDM signal whichis mixed with a 20 GHz sinusoidal wave to realize the sub-carrier modulation (SCM) and then the mixed signals areemployed to drive the DML to generate DSB optical signals.The 2.5Gbit/s OFDM baseband signal can be generatedoffline by Matlab program. The OFDM baseband signal iscalculated offline with Matlab program including mapping215-1 PRBS into 256 16PSK-encoded subcarriers, subse-quently converting the OFDM symbols into time domain byusing IFFT and then adding 32 pilot signals in notch. Guard

Fig. 5 The spectrum of OFDMsignal after transmission overfiber. (a)BTB and (b)After 40 km

Fig. 6 BER curves

162 J Opt (April–June 2014) 43(2):159–164

interval length is ¼OFDMperiod. A low-cost 1,550 nmDMLbiased at 57 mA is employed to generate optical lightwave.This DML has a 3 dB bandwidth higher than 20 GHz. Theoutput power from this DML is 4dBm. The generated DSBoptical signal spectrum is shown in Fig. 3 as inset (i). A Cirand a FBG are used to suppress the optical carrier from thefirst-order sidebands. The FBG has a 3 dB reflection band-width of 0.2 nm. The optical spectrum after the FBG is shownin Fig. 3 as inset (ii). Then the first-order sidebands will betransmitted over 40 km SSMF to the BS after boosted thepower by the EDFA.

At the BS, the optical carrier suppression signals are detectedby an optical receiver which is a high-speed photo-detector witha 3 dB bandwidth of 50 GHz. The 0.4 nm optical bandwidthTOF is employed to suppress the ASE noise. Then the opticalsignals are converted into electrical signals by an optical-electrical (O/E) converter with a 3-dB bandwidth of 40 GHz.The electrical signals are amplified by a narrow-band electricalamplifier (EA) which has a bandwidth of 10 GHz centered at40 GHz. Finally, we employ the 40 GHz electrical LO signaland a mixer to down-convert the electrical mm-wave signal toobtain the downlink baseband signals. The received data areprocessed and recovered off-line with a Matlab program as anOFDM receiver. We employ a bit error ratio (BER) tester tomeasure the downstream signals after down-conversion.

Figure 4(a-c) show the constellation diagrams of the re-ceived signal for different transmission distance over 0 km, 32and 40 km SMF-28 fiber, respectively. Comparing with theback to back (B-T-B) case, the constellation diagrams of thereceived signal for different transmission distance over 32 and40 km show little phase distortion due to the nonlinear modula-tion effect of the DML and fiber dispersion, but the constellationdiagram performance is still good agreement.

Figure 5 a and b show the electrical spectrum of OFDMsignal before and after transmission over 40 km, respectively.The performance is still in good agreement. The measuredBER curve is shown in Fig. 6. The power penalty for theconverted downstream signals which is mainly caused by thenonlinear modulation effect, the frequency chirp of DML andfiber dispersion is smaller than 2-dBm at a BER of 10−2 after40 km SMF-28 transmission, so the effect of fiber dispersionand the nonlinear modulation effect of the DML are small andcan be neglected.

Conclusion

We have investigated and demonstrated a novel scheme togenerate OFDM signals for ROF systems. We employ theFBG because the bandwidth of the optical modulator is largelyreduced and the architecture of the ROF system is simpler.The 40 GHz Optical mm-wave signal were generated by aDML driven by the 2.5Gb/s OFDM signals carried by only

20 GHz RF signals. The power penalty for the converteddownstream signals is smaller than 2-dBm after 40 kmSMF-28 transmission. Our simulation results show that theeffect of fiber dispersion and the nonlinear modulation effectof the DML are small and can be neglected. This novelscheme employs low-cost DML to generate mm-wave signalto carry OFDM signals which is a practical scheme to beapplied for future broadband access networks.

References

1. L. Chen, H. Wen, S. Wen, “A radio-over-fiber system with a novelscheme for millimeter-wave generation and wavelength reuse for up-link connection”. IEEE Photon. Technol. Lett. 18, 2056–2058 (2006)

2. J. Yu, Z. Jia, T. Wang, G.K. Chang, “Centralized lightwave radio-over-fiber system with photonic frequency quadrupling for high-frequency millimeter-wave generation”. IEEE Photon. Technol.Lett. 19, 1499–1501 (2007)

3. C. Park, C. Oh, C. Lee, D. Kim, C.S. Park, A photonic up-converterfor a WDM radio-over-fiber system using cross-absorption modula-tion in a EAM. IEEE Photon. Technol. Lett. 17, 1950–1952 (2005)

4. L. Chen, Y. Shao, X. Lei, H. Wen, S. Wen, A novel radio-over-fibersystem with wavelength reuse for upstream data connection. IEEEPhoton. Technol. Lett. 19, 387–389 (2007)

5. G. Qi, J. Yao, J. Seregelyi, S. Paquet, C. Belisle, Generation anddistribution of a wideband continuous tunable millimeter-wave signalwith an optical external modulation technique. IEEE Trans. Microw.Theory Tech. 53, 3090–3097 (2005)

6. J.E. Mitchell, Performance of OFDM at 5.8 GHz using radio overfiber link. Electron. Lett. 40(21), 1353–1354 (2004)

7. W. Shieh, X. Yi, Y. Tang, Transmission experiment of multi-gigabitcoherent optical OFDM systems over 1000 km SSMF fiber. Electron.Lett. 43(3), 183–184 (2007)

8. H. Bao, Transmission simulation of coherent optical OFDM signalsin WDM systems. Opt. Express 15(8), 4410–4418 (2007)

9. Y. Tang, W. Shieh, X. Yi, R. Evans, Optimum design for RF-to-optical up-converter in coherent optical OFDM systems. IEEEPhoton. Technol. Lett. 19(7), 483–485 (2007)

10. A. Kim, Y.H. Joo, Y. Kim, 60 GHz wireless communication systemswith radio-over-fiber links for indoor wireless LANs. IEEE Trans.Consumer Electron. 50(2), 517–520 (2004)

11. L. Chen, J. He, Y. Li, H. Wen, Y. Shao, C. Huang, L. Hu, Y. Pi, Z.Dong, Y. Li, X. Lei, S. Wen, “Simple ROF configuration to simulta-neously realize optical millimeter-wave signal generation and source-free base station operation”. ECOC 2, 45–46 (2007)

12. T. Kawanishi, K. Higuma, T. Fujita, S. Mori, S. Oikawa, J. Ichikawa,T. Sakamoto and M. Izutsu, “40Gbit/s versatile LiNbO lightwavemodulator,” ECOC, Glasgow, U.K., Paper Th 2.2.5, (2005)

13. T. Sakamoto, T. Kawanishi, M. Izutsu, Continuous-phase frequency-shift keying with external modulation. IEEE J. Sel. Top. Quant.Electron. 12(4), 589–595 (2006)

14. J. Lowery, L. Du Bangyuan, J. Armstrong, Performance of opticalOFDM in ultralong-haul WDM lightwave systems. J Light. Tech. 1,131–138 (2007)

15. W. Jiang, C.T. Lin, A. Ng’oma, P.T. Shih, J. Chen, M. Sauer, F.Annunziata, S. Chi, “Simple 14-Gb/s short-range radio-over-fibersystem employing a single-electrode MZM for 60 GHz wirelessapplication”. J. Lightwave Technol. 28(16), 2238–2246 (2010)

16. J. Armstrong, OFDM for optical communications. J. LightwaveTechnol. 27(27), 189–204 (2009)

J Opt (April–June 2014) 43(2):159–164 163

17. J. Ma, 5 Gbit/s full-duplex radio-over-fiber link with opticalmillimeter-wave generation by quadrupling the frequency of theelectrical RF carrier. J. Opt. Commun. Netw. 3(2), 127–133(2011)

18. Z. Cao, J. Yu, H. Zhou,W.Wang,M. Xia, J.Wang, Q. Tang, L. Chen,WDM-ROF-PON architecture for flexible wireless and wire-linelayout. J. Opt. Commun. Netw. 2(2), 117–121 (2010)

19. J. Yu et al., “Cost-effective optical millimeter technologies and fielddemonstrations for very high throughput wireless-over-fiber accesssystems”. J. Lightw. Technol. 28(16), 2376–2397 (2010)

20. Z. Cao et al., “Reduction of intersubcarrier interference andfrequency-selective fading in OFDM-ROF systems”. J. Lightw.Technol. 28(16), 2423–2429 (2010)

21. W. Jiang, et al., “32.65-Gbps OFDM ROF signal generation at60 GHz employing an adaptive I/Q imbalance correction,” in Proc.36th ECOC, Sep. 2010, pp. 1–3, paper Th.9.B.5

22. Y. Shao, C. Nan, F. Jingyuan, F. Wuliang, “Generation of 16-QAM-OFDM signals using selected mapping method and its application inoptical millimeter-wave access system”. IEEE Photon. Technol. Lett.24(15), 1301–1303 (2012)

23. J. Wang, Y. Guo, X. Zhou, PTS-clipping method to reduce the PAPRin ROF-OFDM system. IEEE Trans. Consum. Electron. 55(2), 356–359 (2009)

24. L. Tao et al., “Spectrally efficient localized carrier distributionscheme for multiple-user DFT-S OFDM ROF-PON wireless accesssystems”. Opt. Express 20(28), 29665–29672 (2012)

164 J Opt (April–June 2014) 43(2):159–164